The ecology of Lake Myvatn and the River Laxá: Variation in space and time

Ecological features of Lake Myvatn and the outflowing River Laxá show a wide range of spatial and temporal variations. The physical division of the lake into three main basins and the variation in chemical composition and temperature of the artesian springs feeding this shallow productive lake have large spatial effects. Variation in groundwater characteristics depends on percolation time and proximity to geothermal sources. Variation in precipitation is evened out by the porous volcanic soil and bedrock and the spring-water discharge is therefore very stable. A pulse of volcanic activity in 1975–1984 (the Krafla Fires) heated the groundwater entering the North Basin of the lake and changed its chemistry. Although much reduced, these effects have not disappeared yet, but overall the impact of the volcanic activity on the biota seemed minimal. Recycling of nutrients through internal loading is important and occurs on various time scales. In winter, when the lake is ice-covered, the topmost 5-cm layer of sediment pore water has a hundredfold concentration of nutrients relative to the overlying lake water. The nutrients are released during the ice-free period by sediment resuspension, diffusion, bioturbation and recycling. In spring, resuspension events sometimes lead to spikes in dissolved phosphorus and nitrogen, but there is little evidence of any major desorption of nutrients from suspended particles during such events later in the summer. In contrast to the stable groundwater, the biota show more or less regular fluctuations with no straightforward correlation with external signals. The most prominent fluctuations, those of the chironomid Tanytarsus gracilentus seem to be driven by interactions between the species and its sediment resources. Fluctuations in other invertebrates could be a consequence of the Tanytarsus cycles due to the large impact this species has on the benthic environment of this detritus-driven ecosystem. Temporal variation in epibenthic chironomids and Cladocera translates into variable production of vertebrate predators (Arctic charr, Salvelinus alpinus, and ducks), body condition and mortality of fish and sometimes into return rates of migrating adult ducks. The waterfowl show large temporal variation on a centennial scale, e.g., the invasion of the tufted duck (Aythya fuligula) which arrived by the end of the 19th century and has by now outnumbered other species. Fluctuations of Cyanobacteria (Anabaena) and the fish Gasterosteus aculeatus (three-spined stickleback) harmonize with the cycles in the benthic community. Palaeolimnological studies indicate that primary production in the South Basin became increasingly benthic as the lake depth was reduced by sedimentation (around 2 mm year^–1). Other trends include a decrease in Tanytarsus and Daphnia and an exponential increase in green algae (Cladophorales, Pediastrum) and associated organisms.     

Lake Myvatn and the River Laxá: An introduction

In July 1999 and May 2001 a group of limnologists and other scientists involved in the research of Lake Myvatn gathered in the village of Skutustadir by the lake for two symposia to discuss and sum up the results based on the research and monitoring that has been conducted there over the past quarter of a century. The ensuing papers now appear in this issue of Aquatic Ecology under the heading 'The Ecology of Lake Myvatn and the River Laxá– Temporal and Spatial Variation'.     

Eutrophication of a Tropical African Impoundment (Lake McIlwaine,Rhodesia)

Lake McIlwaine is an artificially eutrophic impoundment in tropical Africa, which has received sewage effluent since 1952, and has developed troublesome algal blooms. Rapid chemical changes have taken place since about 1960, and the lake is very much richer than other impoundments in the area. Chemical composition of the inflowing rivers shows that sewage effluent is responsible for this eutrophication, and rural runoff is not considered significant. The presence of an anaerobic hypolimnion is a doubtful index of eutrophication as this occurs in tropical lakes, regardless of trophic status. By contrast, the epilimnion of Lake McIlwaine shows permanent oxygen super-saturation, probably due to the high phytoplankton productivity. The lake has a high seasonal inflow and rapid replacement time, factors which could enhance the success of measures to reduce the inflow of sewage effluent.     

Some factors controlling the major ion chemistry of small lakes: Examples from the prairie parkland of Canada

The management of recreational lakes in the prairie region of Canada requires that their physical, chemical and biological setting be clearly defined. This study relates the major ion chemistry of two lakes to significant ion contributions from groundwater, surface runoff and snow. The major ion concentrations in addition are related to several important limnological processes.Lake Wabamun is a 84.6 km² lake with a basin set in Cretaceous bedrock. Groundwater studies in the watershed indicate that rather large quantities of groundwater enter the lake from fractured coal and bedrock units. Hastings Lake has a considerably smaller surface area and is formed in a shallow depression in glacial drift. Generally, smaller quantities of groundwater enter this lake because the glacial drift has a low permeability.Evaluation of the data indicates that relative quantity and quality of groundwater inflow is probably the most important factor controlling the major ion chemistry of the lakes. Other factors are mineral precipitation, freezing out and the timing of major water inflows.     

Physical and chemical features of Lake Xolotlán (Managua)

The hydrochemical character of Lake Xolotlán (Managua) is clearly dominated by sodium bicarbonate, but the lake is also strongly influenced by thermal activities in its coastal and bottom areas which contribute to high concentrations of chlorides and sodium. Lake Xolotlán is endorheic and has a high evaporation rate (2,395 mm per year) which is twice its annual precipitation (1,129 mm). There is an inflow of domestic industrial sewage from the city of Managua. All these factors are subjecting the lake to strong contamination, salinization and eutrofication. The lake can be considered to have an intermediate status between eutrophic and hypertrophic according to measured total-P concentration. Due to high concentrations of some of its ions, the water of Lake Xolotlán cannot be used for irrigation.     

The importance of groundwater flow to the formation of modern thrombolitic microbialites

Modern microbialites are often located within groundwater discharge zones, yet the role of groundwater in microbialite accretion has yet to be resolved. To understand relationships between groundwater, microbialites, and associated microbial communities, we quantified and characterized groundwater flow and chemistry in active thrombolitic microbialites in Lake Clifton, Western Australia, and compared these observations to inactive thrombolites and lakebed sediments. Groundwater flows upward through an interconnected network of pores within the microstructure of active thrombolites, discharging directly from thrombolite heads into the lake. This upwelling groundwater is fresher than lake water and is hypothesized to support microbial mat growth by reducing salinity and providing limiting nutrients in an osmotically stressful and oligotrophic habitat. This is in contrast to inactive thrombolites that show no evidence of microbial mat colonization and are infiltrated by hypersaline lake water. Groundwater discharge through active thrombolites contrasts with the surrounding lakebed, where hypersaline lake water flows downward through sandy sediments at very low rates. Based on an appreciation for the role of microorganisms in thrombolite accretion, our findings suggest conditions favorable to thrombolite formation still exist in certain locations of Lake Clifton despite increasing lake water salinity. This study is the first to characterize groundwater flow rates, paths, and chemistry within a microbialite‐forming environment and provides new insight into how groundwater can support microbial mats believed to contribute to microbialite formation in modern and ancient environments.     

The phosphorus economy of a hypertrophic seepage lake in Scania,south Sweden groundwater influence

The phosphorus dynamics and economy of Lake Bysjön, a hypertrophic seepage lake in Scania, southern Sweden, were investigated during 1973–1977. The mean dissolved inorganic phosphorus concentration (1973–1977) was 580 µg · l^–1. There were no correlations between dissolved inorganic P, total organic P, dissolved organic P, particulate P and phytoplankton biomass. Groundwater inflow and lake water outflow through the ground are the most important factors for maintaining a constant water volume. Groundwater seepage is also important for water quality. Groundwater inflow, together with planktonic activity, keeps the P concentration high in the lake water.     

Population fluctuations of chironomid and simuliid Diptera at Myvatn in 1977–1996

Aquatic dipterans dominate the macrozoobenthos of Lake Myvatn and the River Laxá and are important in the food web. As benthic sampling is time-consuming and expensive, a window trap was developed in order to facilitate the monitoring of the zoobenthos resource. Four window traps were operated at Myvatn and Laxá during 1977–1996. The results yielded population indices for about 20 species of chironomids and one simuliid. Numbers of the most abundant midge species showed close correlations between 2 trap sites on the lake shore, 5 km apart; Cricotopus tibialis did not show such synchrony. At the River Laxá, the dominant blackfly (Simulium vittatum) showed correlation between 2 trap sites, at the outlet and 3 km downstream; while the chironomids, apart from Micropsectra atrofasciata, did not. The dominant midge of Lake Myvatn, Tanytarsus gracilentus showed 3 cyclic oscillations during the 20-year period. The fluctuations were tracked by Micropsectra lindrothi, Procladius islandicus, Orthocladius consobrinus, Psectrocladius barbimanus and Chironomus islandicus. A group of non-cyclic species included Cricotopus sylvestris and Orthocladius oblidens.     

Chemical composition of interstitial water and diffusive fluxes within the diatomaceous sediment in Lake Myvatn, Iceland

The sub-arctic Lake Myvatn is one of the most productive lakes in the Northern Hemisphere, despite an ice-cover of 190 days per year. This is due to relatively high solar radiation, nutrient rich inflow waters, N₂ fixation and internal nutrient loading. In order to define direction and magnitude of diffusive fluxes, soil water samplers were used to collect interstitial water from 25–150 cm depth, from within the diatomaceous sediment at the bottom of Lake Myvatn. Water depth at the sampling site was 225 cm. The pH of the interstitial water ranged from 7.16 to 7.30, while the pH of the lake water was 9.80–10.00. The concentrations of most solutes were similar 16 cm above the bottom of the lake at the sampling site and at the lake outlet. The concentrations of NO₃, S, F, O₂, Al, Cr, Mo, V, U, Sn and Sb were higher in the lake water than in the interstitial water. They will therefore diffuse from the lake water into the interstitial water. The concentrations of orthophosphates, PO₄, and total dissolved P were highest at 25 cm depth, but Co and NH₄ concentrations were highest at 50 to 100 cm depth. Thus they diffuse both up towards the lake bottom and down deeper into the sediments. The concentrations of Na, K, Ca, Mg, Sr, Mn, Li and alkalinity were greater within the sediments than in the lake water and increased continuously with depth. The Si concentration of the interstitial water was higher than in the lake water, it was highest at 25 cm depth and decreased slightly down into the sediments. The concentration gradient was greatest for bicarbonate, HCO₃ ^–, 1.5×10^–7 mol cm^–3 cm^–1, and then in declining order for the solutes with the highest gradient; NH₄, Si, Na, Ca, Mg, -S (diffusion into the sediments), K, PO₄, Cl, Fe and Mn. The estimated annual diffusive flux of PO₄ for Lake Myvatn was 0.1 g P m^–2 yr^–1, about 10% of the total PO₄ input to Lake Myvatn. The H₄SiO₄° flux was 1.3 g Si m^–2 yr^–1, <1% of both the input and the annual net Si fixation by diatoms within the lake and the diffusive flux of dissolved inorganic carbon was 1% of the annual net C fixation by diatoms. Annual diffusive flux of NH₄ ⁺ was 1.9 g N m^–2 yr^–1 similar to the input of fixed N to the lake and 24% of the net N fixation within Lake Myvatn. Thus it is important for the nitrogen budget of Lake Myvatn and the primary production in the lake since fixed nitrogen is the rate determining nutrient for primary production.     

Meromixis and Seasonal Dynamics of Vertical Structure of Lake Uchum (South Siberia)

The seasonal dynamics of the vertical structure of small saline Lake Uchum, located in the steppe arid zone of the south of Siberia (Krasnoyarsk krai), has been studied in detail for the first time. This lake is a meromictic water body. We have revealed a heterogeneous vertical distribution of plankton organisms and a dense population of purple sulfuric bacteria in the redox zone. The taxonomic composition and seasonal dynamics of phyto- and zooplankton are described. Presumably, the meromixis of Lake Uchum is due to the inflow of fresh water to the surface of the saline water body during the rise of its level in the early 20th century, similarly to lakes Shira and Shunet located nearby. The processes of salt displacement into the solution during the formation of ice, as well as the precipitation of salts in the winter, also contribute to the maintenance of permanent stratification. The information on the current state of the lake can be useful for reconstructing the climate by bottom sediments, as well as for creating a model of water quality and investigating the therapeutic properties of lake mud.     

Intraannual and interannual changes in the surface area of a closed lake complex in southwestern Siberia using NOAA images

The Lake Chany complex, located in southwestern Siberia, consists of large shallow lakes with an average depth of about 2.2m. The area of the lake fluctuates with the water level, which is closely related to the amount of inflow and evaporation, as the lake complex is endorheic. Using National Oceanic and Atmospheric Administration (NOAA) satellite images of the ice-free periods from 1999 to 2004, we evaluated the seasonal changes in the lake area and surrounding vegetation of the Lake Chany complex. The maximum lake area was observed in early May and decreased until late July. The lake area in August was about 60% of the maximum. Subsequently, the area tended to increase until early October. Compared to ground-truth data, the areas undergoing seasonal fluctuations in the NOAA images corresponded to vast stands of vegetation (Phragmites australis) that were several kilometers wide. These areas seem to be influenced by the inflow of snow-melt water and reed growth. Although interannual differences in the seasonal change in lake area were not great for the whole lake complex, the isolated Yudinskii Pool showed large interannual differences during summer and autumn, suggesting that monitoring this area using NOAA images will be useful for estimating the interannual fluctuations in the southwestern Siberian environment.     

Assessing Restoration Perspectives of Disturbed Brook Valleys: The Gorecht Area, The Netherlands

Several landscape restoration alternatives were evaluated in the Gorecht area, a cultivated river plain in the northern part of the Netherlands. A landscape analysis was performed to investigate the hydrological functioning of this area. Groundwater composition in the area was assessed by using distribution patterns of indicative plant species. Results proved to be consistent with an interpolation of actual data of the groundwater composition. Groundwater flow was simulated with hydrological models to explain the observed patterns in water chemistry. It appeared that upwelling Ca-rich groundwater is now absent in the area, contrary to the past situation. Because a constant supply of Ca-rich water is an essential condition for mesotraphent fen vegetation, we concluded that under the present conditions the regeneration prospects for these vegetation types are poor. We suggest that the plan to regenerate groundwater-fed fens be abandoned for a plan to create surface-water-fed marshes.     

Sewage effluent discharge and geothermal input in a natural wetland,Tongariro Delta,New Zealand

Effluent from the oxidation ponds of the town of Turangi, south of Lake Taupo, has been discharged into a natural wetland since the 1960s. This has resulted in elevated concentrations of Na⁺, K⁺, Cl⁻ and NH₄⁺-N in both ground and surface water. Increased weed invasion and plant growth, and high heavy metal concentrations (e.g. up to 440 ppm Zn) occur in the vicinity of effluent discharge in the wetland. Element and nutrient concentrations decrease with increasing distance from the effluent inflow point, suggesting that the wetland presently acts as a sink for metals and nutrients. Elevated arsenic concentrations (up to 5800 ppm in peat and 11 400 ppm in the ash fraction) in some of the organic-rich sediment suggest a long-term input by geothermal water originating in the Tokaanu–Waihi field. Increased silt input due to more frequent flooding of the Tongariro river over the past 40 years has resulted in a significant change in stratigraphy (from peat to mud) over much of the wetland.     

Lake Vechten: aspects of its morphometry,climate, hydrology and physico-chemical characteristics

The physiography of Lake Vechten (The Netherlands) is described together with morphometric data. The lake (surface area 4.7 ha; mean depth 6.0 m) consists of two basins with maximum depths of 10.5 and 11.9 m. Meteorological conditions in the region and horizontal groundwater flow play an important role in the renewal time, which is about two years. The lake has in most years a circulation period from November till April but in some years, when the ice cover is prolonged, it is stratified in winter as well. The summer stratification extending from May to the end of October is very stable, with an anaerobic hypolimnion. Eddy conductivity in the stagnant water is calculated. The water transparency is strongly influenced by the presence of algal and bacterial populations and by resuspended particulate matter. Secchi disk depth ranges from 1.8 to 4.5 rn. The 1% of the surface light reaches from 4 to 5 m depth in November and from 8 to 9.5 m depth in May. The ionic composition and nutrient status of the lake are given.     

Geothermal heat flux into deep caldera lakes Shikotsu, Kuttara, Tazawa and Towada

Geothermal heat fluxes into the deepest waters of four caldera lakes were measured. Temperature profiles within the stratification period between July and November 2007 allowed a quantification of the acquired heat. Due to their enormous depth, heat input from the lake bed was locally separated from heat fluxes at the surface. In conclusion, a direct measurement of geothermal heat input could be accomplished. Although enhanced geothermal activity could be suspected in all cases, two lakes showed a geothermal heat flux of 0.29 or 0.27 W/m² (Lake Shikotsu and Lake Tazawa), as found in other regions not affected by volcanism, while both other lakes (Lake Kuttara and Lake Towada) showed a greatly enhanced heat input of 1 or 18.6 W/m², respectively. In conclusion, within our investigated set, all lakes acquired more heat from the underground than the continental heat flux average. Hence, the heat flux into the lakes from the ground was not dominated by the temperature gradient implied by the inner heat of the earth. Other effects like the general temperature difference of deep lake water and the groundwater or local sources of heat in the underground deliver more important contributions. Obviously the flow of water in the underground can play a decisive role in the heat transport into the deep waters of lakes.     

Lake circulation and sediment transport in Lake Myvatn

Lake circulation and sediment transport in Lake Myvatn have been calculated using AQUASEA, a numerical model developed by Vatnaskil Consulting Engineers. The goal of the modelling was to calculate changes in sediment transport within the lake due to changes in lake bathymetry caused by diatomite mining. The model uses the Galerkin finite element method and consists of a hydrodynamic flow model and a transport-dispersion model. The flow model is based on the shallow water equations and the wave equation. The transport model is based on the conservation of mass for suspended sediment. The model was calibrated against measurements performed during the summer of 1992. These included measurements of water elevation, current velocity, wave height, and concentration of suspended sediment. After calibration, the model was run for different mining scenarios to determine their impact on the sediment transport in the lake.     

鄱阳湖湿地地下水埋深及其与典型植被群落分布的关系

地下水位直接影响土壤含水量,进而影响湿地植被生长、分布和演替.本研究以鄱阳湖湿地为研究区,基于2014-2018年地下水位野外定点观测数据,分析湿地地下水年内与年际动态变化特征,构建鄱阳湖湿地地下水数值模型,分析地下水流场空间特征以及地下水埋深分布,结合高斯回归方法分析地下水埋深与典型植被分布之间的关系.结果表明:年内...     

Perennially ice-covered Lake Hoare,Antarctica: physical environment,biology and sedimentation

Lake Hoare (77° 38 S, 162° 53 E) is a perennially ice-covered lake at the eastern end of Taylor Valley in southern Victoria Land, Antarctica. The environment of this lake is controlled by the relatively thick ice cover (3–5 m) which eliminates wind generated currents, restricts gas exchange and sediment deposition, and reduces light penetration. The ice cover is in turn largely controlled by the extreme seasonality of Antarctica and local climate. Lake Hoare and other dry valley lakes may be sensitive indicators of short term (< 100 yr) climatic and/or anthropogenic changes in the dry valleys since the onset of intensive exploration over 30 years ago. The time constants for turnover of the water column and lake ice are 50 and 10 years, respectively. The turnover time for atmospheric gases in the lake is 30–60 years. Therefore, the lake environment responds to changes on a 10–100 year timescale. Because the ice cover has a controlling influence on the lake (e.g. light penetration, gas content of water, and sediment deposition), it is probable that small changes in ice ablation, sediment loading on the ice cover, or glacial meltwater (or groundwater) inflow will affect ice cover dynamics and will have a major impact on the lake environment and biota.     

A hydrologic assessment of the Everglades Nutrient Removal Project, a subtropical constructed wetland in South Florida (USA)

The Everglades Nutrient Removal Project (ENRP), a 1544-ha constructed wetland in south Florida, was intensively monitored throughout its five-year operational history. Water budgets for the ENRP and each of its interior treatment cells were dominated by surface flows (≥85% of inflows; ≥68% of outflows) with smaller contributions from precipitation, evapotranspiration, groundwater flux, and change in storage. The mean water depth, hydraulic loading rate for surface water, and nominal hydraulic retention time for the entire wetland were 0.6 m, 3.1 cm d⁻¹ and 17.7 d, respectively, and were comparable to values anticipated in design. The east flow-way was slightly shallower (0.2 m) and received proportionately more flow (61%) than the west flow-way. The hydrology of other treatment wetlands is often driven by surface flows. All treatment cells in the ENRP were to some extent hydraulically short-circuited. There was net groundwater inflow to the ENRP from Water Conservation Area 1 (WCA-1) resulting from significant head differences between these wetlands. Groundwater outflow to the adjacent farmlands was greatest in Cell 2 and substantially exceeded groundwater inflow. All hydrologic parameters exhibited seasonality to some extent; fluctuation in water depth and groundwater inflows corresponded with the seasonal change in stage in WCA-1. Errors in the ENRP and individual cell water budgets were generally less than 10% and within the range of errors for water budgets from other wetlands.     

Comparison of Recent Siliceous and Carbonate Mat Development on the Shore of Hyper-Alkaline Lake Van and Mt. Nemrut Soğuk Lake,NE Anatolia,Turkey

This article focuses on the molecular geochemistry of previously unreported carbonate- and siliceous-mat assemblages at two adjacent lake-shore environments. The first study area, on the Lake Van shore, is an intersection of hyper-alkaline lake water and seep water discharge, both of which represent an acid-base geochemical barrier. Comparing alkaline lake water with seep water samples collected from a biomineralization site reveals that seep water has an acidic character and is rich in Ca and Si cations. The second study area, Mt. Nemrut, is the largest volcanic caldera located west of Lake Van in eastern Anatolia, and is the only volcano that has erupted in historical time. While the main lithified-mat components of Lake Van's shore environments are Ca-carbonate and diatoms, Na-carbonate and diatom assemblages represent recent biomineralization in the Mt. Nemrut So?uk Lake shores. In particular, the normal habitat for the development of these unique microbial mat assemblages can be changed in a short time by the decreasing seep water discharge into the Lake Van shore environment. A detailed study on these lake shore microbialites, before they are lost, would provide a great opportunity to understand the biogeochemical interactions that produce unique sediment fabrics.